1. Field of the Invention
The present invention is directed to an x-ray image intensifier of the type having an evacuated housing, an input luminescent screen, electron optics and an image sensor attached inside the housing at that side lying opposite the input luminescent screen.
2. Description of the Prior Art
European Application 0 083 240 discloses an x-ray image intensifier of the type for use in x-ray examinations for medical diagnostics. The x-ray image intensifier supplies two-dimensional transillumination exposures in the form of video images. The x-ray quanta are absorbed in a scintillator of the input luminescent screen and are converted into light. The emitted light quanta release electrons in a photo-cathode of the input luminescent screen. These electrons are accelerated in the electrical field of the electron optics and are focused onto an image sensor, which converts the electron image into a video image and supplies corresponding video signals. Using such a known x-ray image intensifier, one directly obtains the electrical image signal for further processing in a digital image system or for video image display. Image sensors in the form of solid-state image pick-ups are usually based on the charge shift principle (CCD) and their standard embodiment is suitable for the documentation of photons in the visible range.
By contrast to photons, electrons leave effects along their entire path through the material. In the electron irradiation of a CCD from the front side, the extremely thin insulating layer would also be affected. This insulating layer, for example, can be composed of SiO.sub.2 and separates the conductive shift structures (gates) from the semiconductor substrate. The demands made of this insulating layer are extremely high because of the high field strengths. Irradiations with charged particles, for example electrons, lead to the formation of quasi-stationary ions and, thus, to the formation of intermediate states (F centers) in the band gap of the SiO.sub.2. These traps lead to an increased dark current and to a deterioration of the charge transfer efficiency. The charging affects also lead to a variation of the shift potential at the gates.